A dental implant is an artificial structure designed to replace a missing tooth. An artificial tooth root part of a dental implant, which is composed of biologically inert material, is made to be embedded and to adhere to the alveolar bone of the missing tooth. An artificial tooth crown part of a dental implant is then connected onto the artificial dental root so as to restore the original function of the missing tooth. A dental implant fixture is the artificial dental root part of a dental implant, which is made to be placed into the alveolar bone.
The dental implant fixtures of the early days had smooth surfaces formed by machining the surface of titanium rods. However, during the last decade researchers have focused on the development of a dental implant fixture having a certain roughness on the surface in order to enlarge the surface area contacting the maxillary or mandibular bone, and thus to achieve a high success rate even under the conditions of insufficient bone quality or quantity, to make it function properly within a short period of time and to be successful with a shorter length. Recently, it has been found that finishing the surface of a dental implant fixture with certain roughness has merits in biological aspects, not only by enlarging the area of mechanical contact with the bone, but also by accelerating the ossecintegration between bone and the dental implant fixture. Thus, the current market is substantially predominated by dental implant fixtures having rough surfaces (Lee, Dong-Han, “What to choose for the surface morphology of implant fixture?”, Journal of Korean Dental Association, 42(5), 2004, published in the website: www.kda.or.kr; and Kim, Young-Kyun and Hwang, Jung-Won, Various Controversies on Dental Implants, Koonja Publishers Company, Seoul, pp. 31-39 (2004)). The issue about whether smooth surface is better or rough surface is better, is no longer a subject of dispute, and the current controversy is concerned in detailed issues such as the method of treatment to obtain rough surface, extent of roughness, appropriate location of the border between the rough and smooth surfaces (hereinafter, referred as R/S border), and the like.
A dental implant fixture having rough surface is more feasible to induce osseointegration and has a larger surface of contact with bone, while a dental implant fixture having smooth surface gives superior results in the reaction with soft tissue covering the bone (Kim and Hwang, ibid.). In particular, when a dental implant fixture having rough surface is exposed to the outside of the soft tissue barrier, which defends against bacterial invasion, the roughness of the surface causes accumulation of plaque (bacterial membrane on the tooth surface) more easily than the smooth surface, thus inducing periodontitis and subsequent loss of alveolar bone, and also increasing the risk of implant failure in the long term (Kim and Hwang, ibid.). Accordingly, most of the dental implant fixtures having rough surfaces are designed to have smooth-surfaced collars/cuffs in the coronal part, where the dental implant fixture is brought into contact with the soft tissue (FIG. 1). In case a dental implant fixture has no smooth surface in the coronal part, the dental implant fixture is usually designed to have a smooth surface contacting with the soft tissue, at the apical end of the dental implant abutment where the abutment joins with the fixture.
Such dental implant fixtures have their placement protocols which say the R/S borders, all designed to be horizontal although their vertical locations vary depending on companies, should be placed at a position lower than the bone crest (the highest part of the alveolar bone). The objects of these placement protocols are to protect the rough surfaces of the dental implant fixtures from bacterial attack by placing the rough surfaces inside of the bone. This serves as a secondary protective barrier to bacterial attack. So, even when the gingiva which serves as a primary protective barrier to bacterial attack has been destroyed by periodontitis, etc., the rough surfaces can be safe inside of the bone. Such placement protocols have been established because exposure of the rough surface of the dental implant fixture to the bacteria in oral cavity could be fatal to the survival of implant (FIG. 2).
When the gingiva is incised and the crestal bone is exposed for the purpose of implant placement, the crestal bone is often inclined downward from the lingual side toward the buccal side, more often than the cases where the crestal bone is flat and horizontal. This is because bone resorption due to disuse atrophy of the maxillary or mandibular bone occurs more actively on the buccal side after loss of a tooth. Such inclination occurs more frequently in the mandibular bone compared to the maxillary bone (Branemark, Per-Ingvar, Ed., Branemark Implant: Surgery, translated by Cho, Sung-Am, Chunji Publisher Company, Seoul, pp. A-14-15 (1997); and Hickey, Judson C., et al., Boucher's Prosthodontic Treatment for Edentulous Patients, Warfel, Darlene, et al., Ed., The C.V. Mosby Company, Missouri, p. 181 (FIG. 9-9) (1985)).
When the crestal bone is inclined as described above, dental implant fixtures having horizontal R/S borders have some portion of their rough surfaces exposed to the outside of the bone crest, when placed without any special treatment (FIG. 3).
In order to prevent such exposure, part of the bone protruded upward (the portion above the dotted line in the left diagram of FIG. 4) should be removed to flatten the crestal bone as illustrated in FIG. 4, or alternatively, guided bone regeneration should be performed at the inclined part of the crestal bone as illustrated in FIG. 5.
Successful implant placement necessitates factors such as provision of sufficient mechanical and physical support. In this regard, placements of longer dental implant fixtures are advantageous. However, the above-described method of removing the protruded part of bone involves placement of a dental implant fixture shortened by a length equal to the length of the removed bone part, and this shortened Length of the dental implant fixture can adversely affect the success of implant placement. In particular, when the distance from the inferior alveolar nerve in the mandibular bone to the bone crest is so short that the length of the dental implant fixture is restricted thereby, removing the part of crestal bone as such is obviously highly unfavorable. The above-described method of performing guided bone regeneration requires significant additional expenses for the use of artificial bone graft and barrier membrane, requires prolonged operation time and patient inconvenience, and increases possibilities for infection and subsequent implant failure.